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Originally Posted by IRBH

I'm still mulling this over.
Your facts are correct - higher suction temp, less dense vapor.
But the enthalpy of the return gas would be higher and that would mean more btu are being pumped .
Not good for discharge temp though.

Well, plotting the system on a ph diagram and doing all of the system calculations will show it.

Originally Posted by syndicated

If you plot a system with a SLHX on a PE diagram, and compare it against the same system without the HX, the increase in NRE is less than the extra superheat gained after the evap. Adding this to the pressure drop across the HX, a large penalty can be seen against the ideal operation of the system.

As was mentioned before, there's no such thing as a free lunch, a classic application of the 2nd law.

it is somewhat more complicated than that.

I will give an example (not real figures)

lets luck at the suction, 6 sh at the evap 21 at the comp, so we could we have 15 of non useful superheat, lets say 100btu for this 15

so we add SLHE, at the evap but after the bulb as in the OP question, we now add the 15 extra superheat by the heat exchange process "useful", because the suction line is warmer it will not absorb the 15 non useful but only a smaller fraction say 5 "33BTU", so at this stage we get a net benifit of 66BTU, but the increased superheat reduces the mass flow through the comp, therefore HOR reduces slightly, dropping the head pressure, reducing power draw.
So when plotting on a PH diagram you need to consider that the evap and cond are fixed surface areas, thus allowance has to be made for this on your plotting data.
Remember that that refrigeration is a "circular" so any change in part of the system, effects all other parts.
The vapour fraction after the TXV is not a free lunch, it is a lunch paid for and not eaten.

If you plot a system with a SLHX on a PE diagram, and compare it against the same system without the HX, the increase in NRE is less than the extra superheat gained after the evap. Adding this to the pressure drop across the HX, a large penalty can be seen against the ideal operation of the system.

As was mentioned before, there's no such thing as a free lunch, a classic application of the 2nd law.

Originally Posted by IRBH

"WASH HIS BEARINGS OUT!!

Where to start? First of all, refrigerant migrated into the crankcase of his semi-hermetic has NOTHING to do with washed out bearings!! It's past the bearings, next stop blown valves/head gaskets!

Refrigerant in the crankcase has EVERYTHING to do with washed out bearings. Not sure what you mean that its past the bearings. The bearings are in the crankcase.

Originally Posted by IRBH

"WASH HIS BEARINGS OUT!!

Where to start? First of all, refrigerant migrated into the crankcase of his semi-hermetic has NOTHING to do with washed out bearings!! It's past the bearings, next stop blown valves/head gaskets!

For goodness sakes read the OP's above post!
Are you a hvac tech, or still in schooling (excellent for you).

http://www.danfoss.com/North_America...AF9EAFE0D.html
The more you speak the more you show your ignorance. Keep talking the world is watching.
Looks like you've got the crankcase heater part down now further your education and geography. New Zealand is in the southern hemisphere.

Originally Posted by IRBH

Barbar... I saw that 10C!
N of 49?

Sorry, Barbar... I was just wondering if you were Canadian, with the celsius.

Originally Posted by IRBH

Barbar... I saw that 10C!
N of 49?

Not quite sure what you mean.
I should clarify my 10C, is more likely to be 11C and certain types of compressors (gas cooler hermetic) especially ones who do not have additional cooling, head fans, liquid or vapour injection. Gas cooled semi's normally higher around 20C, direct into the head higher still. Also depends on the refrigerant. LT normally meaning a compressor with a high compression ratio.
You should always check against the manufactures working envelope.

Barbar... I saw that 10C!
N of 49?

Originally Posted by VTP99

I doubt very much that one start-up per year is going to wash his bearing out. But yet you have no concern what his superheat is at his compressor. Also I have more faith then you that the
OP would no to wait till the box was at near set temperature to check his superheat. Wow and apparently your experience does not no that a ZP power element IS a MOP ( maximum operating pressure ) Please continue to enlighten me. LOL

"WASH HIS BEARINGS OUT!!

Where to start? First of all, refrigerant migrated into the crankcase of his semi-hermetic has NOTHING to do with washed out bearings!! It's past the bearings, next stop blown valves/head gaskets!

For goodness sakes read the OP's above post!
Are you a hvac tech, or still in schooling (excellent for you).

Originally Posted by bunny

There is no free lunch...


When the suction vapor temperature is increased, it becomes less dense. Compressor capacity is decreased as the vapor density (lb/cu ft) decreases. The compressor is a volume pump, so while it's pumping the same volume, pumping the less dense vapor reduces the mass flow rate.

Liquid to suction heat exchangers reduce liquid temperature, but at the expense of higher suction vapor temperature. Any increase in efficiency gained by the lower liquid temperature is offset by the decrease in compressor capacity from the higher suction vapor temperature.

Classic case of robbing Peter to pay Paul..

I'm still mulling this over.
Your facts are correct - higher suction temp, less dense vapor.
But the enthalpy of the return gas would be higher and that would mean more btu are being pumped .
Not good for discharge temp though.

Originally Posted by IRBH

I would still urge you to install heaters on seasonal systems! Nothing to do with cold ambient- it's strictly to "boil off" the refrigerant that migrates to the oil.
Very glad to see that you did not follow Bla Bla Bla's advice and start cranking the TXV just because you did not see 20 SH at comp or 5 SH at the coil when you were 40deg shy of target temp! Any experienced tech would know that the txv is just an orifice under these conditions!
Oh, well his young eyes caught my typo of "zp" instead of mop.

I doubt very much that one start-up per year is going to wash his bearing out. But yet you have no concern what his superheat is at his compressor. Also I have more faith then you that the
OP would no to wait till the box was at near set temperature to check his superheat. Wow and apparently your experience does not no that a ZP power element IS a MOP ( maximum operating pressure ) Please continue to enlighten me. LOL

Phmtech, you should carry a heat lamp to warm the crnkcase of these dormant units for a couple hours. And beware any that show "over the glass" oil levels. That probably isn't oil!

Originally Posted by Phmteck

Thank you for your input. I have run into problems starting one particular freezer at another of our schools. I didn't pump it down when shutting off. And then when it came to restart it I flipped the breakers in another room. Went to where the compressor was and it was not sounding good. So I shut the disconnect off at the compressor after climbing a 10' ladder, but it was too late. It had already blown the gasket out between the valves. Replaced valve plate and gasket. And being as sharp as a tack. I turned it on again. Guess what! It did it again. Blowing the gasket in the same place even though it had only run 10-15 seconds.
After correcting my mistake again. I took and locked the lssv and slowly opened it as box cooled down. Worked like a charm.

Lesson learned the hard way. That was before I found this wonderful site.

None of our equipment has crankcase heaters on them. All our condensing units are located inside. Either on a mezzanine above unit or on a rack in another room. Most of the times the temp in those areas are above 65 and below 90*

I would still urge you to install heaters on seasonal systems! Nothing to do with cold ambient- it's strictly to "boil off" the refrigerant that migrates to the oil.
Very glad to see that you did not follow Bla Bla Bla's advice and start cranking the TXV just because you did not see 20 SH at comp or 5 SH at the coil when you were 40deg shy of target temp! Any experienced tech would know that the txv is just an orifice under these conditions!

Oh, well his young eyes caught my typo of "zp" instead of mop.

Originally Posted by Phmteck

I have done no adjusting of the txv. Just to let you know. Seeing how its been fine for the last couple years. I am concerned about having the leak that caused me to be low in the first place.

So with this SLHE. 1. It should keep feeding liquid to my txv even if system is a slightly undercharged. 2. My suction pressures my be a little higher.

When I get back to work Monday I will see what going on with this freezer and get you all some numbers to work with ,and hopefully find where my leak was. Who knows maybe the thing will be at temp. (Wishful thinking)

Since the unit it was low should I be worried about the 404a fractionating over the summer and messing up my effective blend of the refrigerant?

Do not concern yourself with R404a fractionating, not an issue.
A SLHE will not mask shortage of refrigerant. Again depending upon the specifics of a system, system capacity may reduce (the comp), but net refrigeration (what you are trying to cool) may increase.
"Note" there is quite a discussion on placing SLHE at the condensing unit, especially with R4** refrigerants, because other ease at which flash gas is produced, and the difficulty in re-liquifing because of glide issues.
If just designed as a holding freezer, you can not expect to see normal running conditions during pull down. You would see higher suction pressures, higher head pressures, and high superheat, during pull down.
I think shutting down you plant has been covered "pump it down", i also recommend at some stage in the "off" that a dehumidifier be placed inside the box, to draw any free water from the insulation.

Originally Posted by IRBH

Ptech, we do lots of seasonal shutdowns -fur farm, hunting lodges, wild game processors.
The "breaker off" is not a good shut off procedure! Turn the stat up to initiate a full pumpdown. We then close the LL valve.

Seasonal compressors MUST be equipped with CCH. With stat set hi, units are powered 24hr before turn on.
Problem is refrigerants' great affinity for oil...it will find a way of migrating into the compressor & there go the valves on cold startup.

Thank you for your input. I have run into problems starting one particular freezer at another of our schools. I didn't pump it down when shutting off. And then when it came to restart it I flipped the breakers in another room. Went to where the compressor was and it was not sounding good. So I shut the disconnect off at the compressor after climbing a 10' ladder, but it was too late. It had already blown the gasket out between the valves. Replaced valve plate and gasket. And being as sharp as a tack. I turned it on again. Guess what! It did it again. Blowing the gasket in the same place even though it had only run 10-15 seconds.
After correcting my mistake again. I took and locked the lssv and slowly opened it as box cooled down. Worked like a charm.

Lesson learned the hard way. That was before I found this wonderful site.

None of our equipment has crankcase heaters on them. All our condensing units are located inside. Either on a mezzanine above unit or on a rack in another room. Most of the times the temp in those areas are above 65 and below 90*

I have done no adjusting of the txv. Just to let you know. Seeing how its been fine for the last couple years. I am concerned about having the leak that caused me to be low in the first place.

So with this SLHE. 1. It should keep feeding liquid to my txv even if system is a slightly undercharged. 2. My suction pressures my be a little higher.

When I get back to work Monday I will see what going on with this freezer and get you all some numbers to work with ,and hopefully find where my leak was. Who knows maybe the thing will be at temp. (Wishful thinking)

Since the unit it was low should I be worried about the 404a fractionating over the summer and messing up my effective blend of the refrigerant?

Originally Posted by Frostmonkey74

I'm glad someone chimed in with the truth. Suction line heat exchangers in my experience are common place in large systems ie supermarket refrigeration .

True, and usually markets have mechanical subcoolers as well in my experience.

Originally Posted by Frostmonkey74

I'm glad someone chimed in with the truth. Suction line heat exchangers in my experience are common place in large systems ie supermarket refrigeration .

True and probably due to the long liquid lines increasing the potential for flash gas.

Originally Posted by barbar

There is quite a bit crap written.
LT compressors generally can not have more than 10C (16f) superheat at the compressor, you can check this with this against the manufactures working envelope.
The liquid/suction line heat exchanger, can increase the system performance, and is related to the specifics of the system (refrigerant and compression ratio), If the TXV bulb is fitted after the SLHE, then it can increase the capacity of the evap by approx 10%.
The heat exchanger also coverts most of the gained superheat into useful superheat not non useful superheat.
Yes it does help in ensuring a liquid head at the TXV inlet.
if the evap is a high pressure drop then some liquid sub cooling can reduce this drop increasing the LMTD, basically lifting the outlet suction pressure slightly.
They are not used as often as they should because of the associated cost.
Your sight glass is an aid to charging.
freezer room are normally designed to maintain a temp, so pull down can take quite some time (excessive load)
You also may have an MOP valve, which restricts the refrigerant mass flow, to ensure head pressure does not rise excessively. This can give false indications especially very high suction superheat on start up. (you see this more often just after a defrost)
As a side note, it is quite common that freezer rooms that are insulated with expanded polystyrene, can absorb quite a bit of water vapour (which turns to ice) during it normal working life. So by turning it off, the ice will melt. So when you turn it on back on you tend to find a reasonble pull down time to -6C, it then stops there for quite a while as the water re-freezes (remember water SHC 1BTU/Lb/f, verses latent heat of water/ice which is 144Btu/lb) this can add quite a load to the refrig system.

I'm glad someone chimed in with the truth. Suction line heat exchangers in my experience are common place in large systems ie supermarket refrigeration .

Originally Posted by IRBH

Biggest error in refrigeration... pissing around with SH adjustment in a previously working, 40deg above target temp, probable ZP element TXV.

Ck for restriction, confirm element is controlling, but stay the heck away from the adjustment when at +40f !!

Blah Blah Blah... Maybe you should reread my post. Never said he should adjust the superheat unless he did not have 20* at the compressor. I said he should look for a superheat of 4-6* at the evaporator. How would he know otherwise if he is getting complete efficiency out of his evaporator ? Also in case you didn't know a ZP element has noting to do with the evaporator superheat. Don't underestimate the OP knowledge.